In particular, the invention is directed to such sterilization in an installation of the type comprising a tunnel closed at both ends, and, from the upstream end towards the downstream end of which are provided: a heating station for heating a thermoplastic band held laterally by clamping members provided on a conveying system such as endless chains; a thermoforming station for forming containers from the thermoplastic band; a metering/dispensing device for the sterile product, the metering device having at least one row of outlet tubulures penetrating into the tunnel; a container closing and sealing station preferably provided downstream of the tunnel; as well as supply units supplying sterile gas under pressure, issuing at several locations in the tunnel, of which the base is normally obturated by the thermoplastic band.
In a known method for sterilization of such an installation the base of the tunnel is temporarily obturated by a heat-resistant sheet, such as a sheet of metal such as aluminium, extending from the upstream end of the tunnel to beyond the downstream end thereof and joined to the thermoplastic band whose face turned towards the upper part of the tunnel is sterile and protected against pollution. A hot sterilizing fluid is then circulated separately through the tunnel and through the metering/dispensing device, and, after the sterilization of the tunnel and of the metering/dispensing device, the heat-resistant sheet and the thermoplastic band are moved forward until the whole base of the tunnel is formed by a length of said thermoplastic band while the tunnel continues to be scavenged with a sterile gas, having a pressure sufficient to allow it to escape through interstices existing between the lateral walls of the tunnel and the edges of the heat-resistant sheet or the thermoplastic hand projecting laterally from said tunnel.
This type of method, which is known for example from U.S. Pat. No. 4,165,594, requires a tunnel with walls, inside which a very hot fluid can be circulated, said fluid heating the inside face of the tunnel to a suitable sterilizing temperature. It is also possible to sterilize the inside face of the tunnel and all the elements issuing therein, with a very hot sterilizing gas, reaching for example a temperature of about 200.degree. C. But since the sterilizing fluid cannot circulate through the lower part of the lower end of the metering/dispensing device opening out into the tunnel, said part can only be heated very slowly, hence involving very long sterilizing periods, especially as the tunnel and the metering/dispensing device then have to be cooled down to a temperature of about 30.degree. to 35.degree. C. before the metering and dispensing of the sterile product into the sterile containers of the sterile thermoplastic band can begin.